A known flight control surface controller uses a plurality of hydraulic servomechanism systems that control fight control surfaces of an aircraft such as an elevator, a rudder, and an aileron. To provide the flight control surface controller with redundancy so that when a failure, such as a decrease in the pressure supplied from a hydraulic pressure source, occurs in one hydraulic servomechanism system, the flight control surface controller uses another hydraulic servomechanism systems to control the flight control surfaces.
Japanese Laid-Open Patent Publication No. 6-144385 describes an example of a conventional flight control surface controller including a plurality of hydraulic servomechanisms. Each hydraulic servomechanism includes a hydraulic valve located between a hydraulic actuator, which drives a flight control surface, and a switch valve, which allows for bypass communication of several hydraulic chambers of the hydraulic actuator. The hydraulic valve includes a check valve, which defines a closed oil channel (closed hydraulic pressure section) at the side of the hydraulic actuator, and a relief valve, which bypasses the check valve and releases oil from the hydraulic actuator side toward the switch valve. The check valve is closed by hydraulic pressure of the closed oil channel. The check valve opens when pushed by a valve opening component with a force that is greater than the hydraulic pressure of the closed oil channel.